In order to increase the crop yield, it is conventional to apply fertilizer to the ground. On a commercial farming scale, a fertilizer applicator is moved across a field to be fertilized, and liquid or dry fertilizer is removed from the applicator and applied to the ground. This type of applicator, when first introduced, had no mechanism for adjusting the fertilizer application rate during application, e.g. in response to changes in depth of soil. The fertilizer rate could only be adjusted while the applicator was stationary, and the operator had to guess as to the depth of soil in order to determine the optimum application rate. If there were wide variations in depth of soil over the field, there may be areas in which the concentration of fertilizer in the soil was too low, with the result that the crop yield was lower than the potential yield, and areas in which the concentration of fertilizer was too high, with the result that the crop was damaged and no yield was provided.
Recently, an applicator for liquid fertilizer has been produced with a control system that allows the operator to adjust the rate of application of fertilizer as the applicator moves over the field to be fertilized. The control system includes a control console that is mounted in the tractor driver's cab and provides a signal to a control valve for determining the application rate. The tractor driver is therefore able to adjust the rate of application of fertilizer as the applicator is towed over the field. In order to use this capability to match the rate of application of fertilizer to the soil depth, it is necessary to know the manner in which the soil depth changes over the area of the field. Hitherto, it has been conventional to measure soil depth by manually probing the soil at various locations over the field. A given field to be fertilized might be a mile or more square, and if the soil is probed every hundred yards this implies that several hundred probes must be made in order to enable adjustment of the control valve every hundred yards. Therefore, this technique involves a great deal of labor. Moreover, the depth of soil can change significantly over distances that are much shorter than 100 yards. Accordingly, the information yielded by manual probing is not accurate enough to avoid under application and over application of fertilizer.